Richmond police say 18-year-old Jose Carlos Montano was arrested around 5 p.m. Thursday outside his San Pablo home.
Richmond police Lt. Mark Gagan describes Montano as having played a "significant role in the rape of the victim." Gagan says Montano was being held on suspicion of rape, rape in concert with force and other charges.
Montano was being held on $1.3 million bail.
The arrest of Montano comes after three other teen suspects appeared for the first time in court Thursday.
A 19-year-old and a 21-year-old have also been arrested in the case.
Police say the victim was attacked after leaving a homecoming dance at Richmond High School Saturday night.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
RICHMOND, Calif. (AP) — Wearing bulletproof vests, three teen suspects appeared for the first time in court Thursday on charges of gang raping a 15-year-old girl outside a high school dance while as many as two dozen people watched without calling police.
Defendant Cody Ray Smith, 15, pleaded not guilty, while Ari Abdallah Morales, 16, and Marcelles James Peter, 17, did not enter pleas during their arraignment in Contra Costa County Superior Court.
Peter's aunt, Monica Peter, said before the hearing that her nephew had told her he was only a bystander and did not participate in the attack.
Asked why he didn't do anything to stop it, she said he feared "he would get his ass kicked."
Sheriff's spokesman Jimmy Lee could not explain why the defendants wore the protective vests. But prosecutor Dara Cashman said her office had been getting a tremendous number of calls and e-mails about the case, mostly "expressions of outrage from all over the country."
Morales was sporting a black eye in court from a separate incident, authorities said.
Morales' lawyer, Ernie Castillo, said he was still learning details about the case.
"I don't know what limited role, if any, my client played," Castillo said. "This was a tragic, horrible incident that happened to this young woman."
Reports have indicated onlookers laughed and joked during the attack. Police would not comment on rumors the episode had been recorded on cell phones.
Smith, Morales, and Peter were each charged with rape in concert and sexual penetration with a foreign object. They were being held without bail and could be eligible for life in prison, if convicted.
They were charged as adults because of the severity of the crime, which showed "callousness and viciousness," prosecutor Dara Cashman said.
"It's just very cold," she said after the hearing.
Another suspect, 19-year-old Manuel Ortega, was arraigned separately but did not enter a plea. He is charged with rape, robbery and assault causing great bodily injury. He is being held on $1.2 million bail.
Salvador Rodriguez, 21, also remained jailed but has not been charged.
Police said the victim left the homecoming dance at Richmond High School and was walking to meet her father for a ride home when a classmate invited her to join a group drinking in the school courtyard.
The girl had consumed a large amount of alcohol by the time the assault began, police said. The girl's father tried to call her cell phone but no one answered.
Police received a tip about a possible assault on campus from a former student, who heard two males bragging about it. Officers found the girl semiconscious and naked from the waist down near a picnic table.
She was released from a hospital Wednesday.
Police are still looking for more suspects in the Saturday night attack and believe as many as 10 people ranging from 15 to the mid-20s may have attacked the girl for more than two hours in a dimly lit area.
A $20,000 reward is being offered for information.